Commercial allergenic extracts are complex mixtures of molecules of which only a few have allergenic activities. Current methods of potency measurements of allergenic extracts with RAST inhibition or ELISA competition is based on serum from allergic patients. Sandwich ELISA utilizes two monoclonal antibodies that recognize different epitopeson a specific allergen. This method is specific, sensitive, fast, and reproducible. It measures the quantity of a specific allergen in the mixture.The contents of Fel d I, Der fI/fII and Der pI/pII in CBER reference extracts of cat, mites have been determined by sandwich ELISA. There is no difference in sensitivity, linearity, or accuracy when either purified allergens or CBERreference extracts are used as standard in the assay. Correlation between current Fel d I standardization method (RID) and sandwich ELISA gave a value of 0.938(n=14). Quantitation of commercial mite extracts using either CBER reference orpurified allergens as standard are comparable. Sandwich ELISA was also used to assess the stability of specific allergens in allergenic extracts. Stability study was conducted at 4oC, room temperature, 37oC and 50oC. Fel d I was stable at all temperatures over a period of one year. Der fI /f II maintained their stability at 4oC and room temperature, but decreased steadily in concentration at 37oC, and disappeared after 3 month at 50oC. Der p I was stable at 4oC and not at higher temperatures. The concentration of Der p II was reduced at all temperatures. In collaboration with Dr. M. C. Kuo at Immulogic and Dr. A. Karpas at CBER, monoclonal antibodies against Fel d I have been produced and screened. Evaluation of the monoclonal antibodies for Fel d I sandwich ELISA is underway.